This invention relates to magnetic field sensors and, more particularly, but not by way of limitation, relates to magnetic field sensors for detecting the presence of weak magnetic fields, such as magnetic fields weaker than the earth's magnetic field.
Various types of magnetic field sensors or detectors are known and used in many industries for many purposes, including motion detection and flow measurement. One of the technological barriers in the development of magnetic field sensors has been the earth's magnetic field. In order to reduce power consumption and increase sensitivity and accuracy of magnetic field sensors, it is desirable to operate at or detect magnetic fields of weak magnitude. The earth's magnetic field has presented a barrier in that it has been necessary to operate at magnetic field magnitudes sufficiently above the earth's magnetic field to avoid interference and false signals.
For example, turbine meters are commonly used to measure the flow of fluids, including gases. Because of the low mass characteristic of gases, the turbine meters used to measure gas are very delicate and sensitive. Typically, gas flow turbine meters have ferrous blades and magnetic sensors are used to detect the motion of the blades. In order to avoid interference by the earth's magnetic field, the magnetic sensors typically create a magnetic field much stronger than the earth's magnetic field.
For example, eddy current sensors are available which will detect the motion of turbine meter blades without interfering with the blades' motion. In order to create a "sensing" magnetic field which is of sufficient strength to avoid interference by the earth's magnetic field, the eddy current sensors typically require approximately 10 milliamps of electrical current and therefore require a continuous power supply. An alternative to eddy current sensors is the use of a sensor having a permanent magnet, such as a magnetic reluctance type sensor. The amplitude and frequency of the output signal of the magnetic reluctance sensors are functionally related to the rotational speed of the turbine meter blades and, therefore, in order to measure low flow rates, the magnetic reluctance sensor needs to create a relatively large flux density (on the order of 82 gauss). This flux density tends to lock the ferrous blades of the turbine meter in position until a minimum flow having sufficient energy to overcome the magnetic field is present.
Therefore, there is a need for a magnetic flow sensor which will detect weak magnetic fields (on the order of magnitude of the earth's magnetic field) at low levels of power consumption. There is also a need for such a sensor which will not interfere with the operation of delicate instruments, such as gas flow turbine meters.